In recent years, research and development have been extensively conducted on light-emitting elements using electroluminescence (EL). In a basic structure of such a light-emitting element, a layer containing a light-emitting substance is provided between a pair of electrodes. By applying voltage to this element, light emission can be obtained from the light-emitting substance.
Since the above light-emitting element is a self-luminous type, a light-emitting device using this light-emitting element has advantages such as high visibility, no necessity of a backlight, low power consumption, and the like. Further, such a light-emitting element also has advantages in that the element can be formed to be thin and lightweight and that response time is high.
The light-emitting device having the light-emitting element can have flexibility and impact resistance in addition to its thinness and lightness and thus has been expected to be applied to a flexible substrate. The light-emitting element is applied not only to the light-emitting device but also to a semiconductor device or the like which functions by utilizing semiconductor characteristics.
As a method for manufacturing semiconductor device using a flexible substrate, a technique in which after a semiconductor element such as a thin film transistor is formed over a base material such as a glass substrate or a quartz substrate, the semiconductor element is transposed from the base material to another base material (for example, a flexible base material) has developed. In order to transpose the semiconductor element to another base material, a step for separating the semiconductor element from the base material that is used for forming the semiconductor element is necessary.
For example, Patent Document 1 discloses a separation technique using laser ablation, which is described below. A separation layer formed of amorphous silicon or the like is formed over a substrate, a layer to be separated which is formed of a thin film element is formed over the separation layer, and the layer to be separated is bonded to an object to which the layer to be separated is transposed by a bonding layer. The separation layer is ablated by laser light irradiation, so that separation of the separation layer is generated.
In addition, Patent Document 2 discloses a technique in which separation is conducted by physical force such as human hands. In Patent Document 2, a metal layer is formed between a substrate and an oxide layer and separation is generated at an interface between the oxide layer and the metal layer by utilizing weak bonding between the oxide layer and the metal layer at the interface, so that a layer to be separated and the substrate are separated from each other.
In Patent Document 2, an interlayer insulating film is formed over the light-emitting element including an anode, an organic light-emitting layer, and a cathode, and the interlayer insulating film is bonded to a supporting member using a bonding layer. Then, separation is performed at the interface between the oxide layer and the metal layer, and the layer to be separated including the light-emitting element is bonded to a film substrate using the bonding layer, so that a light-emitting device using a flexible substrate is manufactured.